Influenza A viruses cause a highly contagious acute respiratory disease. Seasonal influenza epidemics cause >300,000 deaths/yr worldwide, while pandemics elicited devastating loss of life in the 20th century, and may do so in the 21st. Currently, however, essential mechanisms underlying development of lung dysfunction and injury in influenza or any other pulmonary viral infection are poorly defined. Both respiratory syncytial virus (RSV) and influenza virus infection of BALB/c mice induce increased channel-mediated release of the nucleotides UTP and ATP into the bronchoalveolar lining fluid (BALF), and, in both infections, elevated BALF UTP contributes to development of lung edema and hypoxemia. RSV infection in BALB/c mice causes only mild disease while influenza causes more severe disease with many of the pathophysiologic features of acute lung injury, which is also seen in lethal pandemic or epidemic influenza in man. Importantly, following influenza, but not RSV, infection, elevated ATP release into BALF is accompanied by increased activation of A1-subtype adenosine receptors (AdoR) by the ATP degradation product adenosine. Adenosine stimulation of neutrophil A1-AdoR plays a significant role in the initiation and promotion of acute lung injury. The central hypothesis of this proposal is that influenza stimulates de novo ATP synthesis and release from infected ATII cells, that released ATP is metabolized to adenosine at an accelerated rate (due to increased ectonucleotidase CD73 activity), and that, by its effects on A1-AdoR, adenosine in the BALF plays a pivotal role in inducing lung injury in influenza. This hypothesis will be tested in 3 Specific Aims: 1) To determine temporal effects of influenza infection on pulmonary nucleotide metabolism, changes in nucleotide metabolism following influenza infection of MLE-12 cells and purified alveolar type II (ATII) cells, as well as FACS-sorted ATII cells from C57Bl/6 or SP- C/GFP mice will be measured, together with the impact of influenza infection on the BALF nucleotide profile, and the role of ERK MAP kinase in inducing nucleotide synthesis after influenza infection. 2) To determine the role of CD73 in influenza pathogenesis, effects of influenza infection on respiratory epithelial cell CD73 expression and activity will be measured; the impact of pharmacologic blockade of CD73 on influenza pathogenesis in C57BL/6 mice will be determined; and the outcome of influenza infection in CD73-knockout mice will be investigated. 3) To determine the role of adenosine/A1-AdoR receptor signaling in influenza pathogenesis, the effect of influenza-induced NF-kB activation on A1-AdoR expression in ATII cells and neutrophils will be measured; the impact of pharmacologic A1-AdoR blockade on influenza pathogenesis in C57BL/6 mice will be determined; and the outcome of influenza infection in A1-AdoR-/- knockout mice, or in A1- AdoR-/- bone marrow-chimeric mice will be investigated. Completion of these objectives will lead to increased understanding of the role of adenosine in the pathogenesis of influenza-associated lung injury, and permit determination of its potential as a target for therapeutic intervention to combat influenza-induced lung damage.